.bzl Style Guide

Starlark is a language that defines how software is built, and as
such it is both a programming and a configuration language.

You will use Starlark to write BUILD files, macros, and build rules. Macros and
rules are essentially meta-languages - they define how BUILD files are written.
BUILD files are intended to be simple and repetitive.

All software is read more often than it is written. This is especially true for
Starlark, as engineers read BUILD files to understand dependencies of their
targets and details of their builds.This reading will often happen in passing,
in a hurry, or in parallel to accomplishing some other task. Consequently,
simplicity and readability are very important so that users can parse and
comprehend BUILD files quickly.

When a user opens a BUILD file, they quickly want to know the list of targets in
the file; or review the list of sources of that C++ library; or remove a
dependency from that Java binary. Each time you add a layer of abstraction, you
make it harder for a user to do these tasks.

BUILD files are also analyzed and updated by many different tools. Tools may not
be able to edit your BUILD file if it uses abstractions. Keeping your BUILD
files simple will allow you to get better tooling. As a code base grows, it
becomes more and more frequent to do changes across many BUILD files in order to
update a library or do a cleanup.

Do not create a macro just to avoid some amount of repetition in BUILD files.
The DRY principle
doesn’t really apply here. The goal is not to make the file shorter; the goal is
to make your files easy to process, both by humans and tools.

Naming convention

Top-level private values start with one underscore. Bazel enforces that
private values cannot be used from other files. Local variables should not
use the underscore prefix.

Line length

As in BUILD files, there is no strict line length limit as labels can be long.
When possible, try to use at most 79 characters per line.

Keyword arguments

In keyword arguments, spaces around the equal sign are optional, but be
consistent within any given call. In general, we follow the BUILD file
convention when calling macros and native rules, and the Python convention for
other functions, e.g.

Boolean values

Prefer values True and False (rather than of 1 and 0) for boolean values
(e.g. when using a boolean attribute in a rule).

Use print only for debugging

Do not use the print() function in production code; it is only intended for
debugging, and will spam all direct and indirect users of your .bzl file. The
only exception is that you may submit code that uses print() if it is disabled
by default and can only be enabled by editing the source – for example, if all
uses of print() are guarded by if DEBUG: where DEBUG is hardcoded to
False. Be mindful of whether these statements are useful enough to justify
their impact on readability.

Macros

A macro is a function which instantiates one or more rules during the loading
phase. In general, use rules whenever possible instead of macros. The build
graph seen by the user is not the same as the one used by Bazel during the
build - macros are expanded before Bazel does any build graph analysis.

Because of this, when something goes wrong, the user will need to understand
your macro’s implementation to troubleshoot build problems. Additionally, bazel
query results can be hard to interpret because targets shown in the results
come from macro expansion. Finally, aspects are not aware of macros, so tooling
depending on aspects (IDEs and others) might fail.

A safe use for macros is leaf nodes, such as macros defining test permutations:
in that case, only the “end users” of those targets need to know about those
additional nodes, and any build problems introduced by macros are never far from
their usage.

For macros that define non-leaf nodes, follow these best practices:

A macro should take a name argument and define a target with that name.
That target becomes that macro’s main target.

All other targets defined by a macro should have their names preceded with a
_, include the name attribute as a prefix, and have restricted
visibility.

All the targets created in the macro should be coupled in some way to the
main target.

Keep the parameter names in the macro consistent. If a parameter is passed
as an attribute value to the main target, keep its name the same. If a macro
parameter serves the same purpose as a common rule attribute, such as
deps, name as you would the attribute (see below).

When calling a macro, use only keyword arguments. This is consistent with
rules, and greatly improves readability.

Engineers often write macros when the Starlark API of relevant rules is
insufficient for their specific use case, regardless of whether the rule is
defined within Bazel in native code, or in Starlark. If you’re facing this
problem, ask the rule author if they can extend the API to accomplish your
goals.

As a rule of thumb, the more macros resemble the rules, the better.

Rules

Rules, aspects, and their attributes should use lower_case names (“snake
case”).

Rule names are nouns that describe the main kind of artifact produced by the
rule, from the point of view of its dependencies (or for leaf rules, the
user). This is not necessarily a file suffix. For instance, a rule that
produces C++ artifacts meant to be used as Python extensions might be called
py_extension. For most languages, typical rules include:

*_library - a compilation unit or “module”.

*_binary - a target producing an executable or a deployment unit.

*_test - a test target. This can include multiple tests. Expect all
tests in a *_test target to be variations on the same theme, for
example, testing a single library.

*_import: a target encapsulating a pre-compiled artifact, such as a
.jar, or a .dll that is used during compilation.

Use consistent names and types for attributes. Some generally applicable
attributes include:

runtime_deps: label_list: runtime dependencies that are not needed
for compilation.

For any attributes with non-obvious behavior (for example, string templates
with special substitutions, or tools that are invoked with specific
requirements), provide documentation using the doc keyword argument to the
attribute’s declaration (attr.label_list() or similar).

Rule implementation functions should almost always be private functions
(named with a leading underscore). A common style is to give the
implementation function for myrule the name _myrule_impl.